Non-alcoholic fatty liver disease is a growing public health concern, particularly in youth. At least 8% of U.S. adolescents have an abnormally-elevated liver enzyme alanine aminotransferase (ALT), a biomarker for NAFLD. Because elevated ALT levels and NAFLD are associated with all-cause and cardiovascular disease mortality in adults, it is a public health priority to initiate interventions at an early age. Increased participation in physical activity and greater cardiorespiratory fitness are associated with lower ALT levels in adults;consequently, experts recommend physical activity as a first line of defense for prevention and treatment of NAFLD. However, no studies have examined this relationship in youth. The proposed study would be the first to examine the associations of physical activity, cardiorespiratory fitness and ALT levels in a large subset of adolescents that are representative of the U.S. population. The proposed study has 2 Aims: the first Aim will be to examine the association between physical activity and ALT in adolescents, and to determine if these relationships are independent of BMI, waist circumference, age, gender, race, ethnicity and cardiorespiratory fitness. Data from adolescents aged 12-19 years whose physical activity was measured by accelerometry will be downloaded from the National Health and Nutrition Examination Survey (NHANES) and will be analyzed to test the hypotheses that: 1) greater amounts of total physical activity per day are associated with lower ALT levels in adolescents;2) greater number of average minutes per day spent in moderate-to- vigorous physical activity (MVPA) are associated with lower ALT levels;and 3) adolescents who meet the national recommendation of obtaining 60 min/day of MVPA for 5 or more days per week will have lower ALT levels than adolescents who do not meet the recommendation. The second Aim will be to examine the associations between cardiorespiratory fitness and ALT in adolescents, and to determine if these relationships are independent of BMI, waist circumference, age, gender, race, ethnicity and physical activity. Data on cardiorespiratory fitness in adolescents 12-19 years of age will be downloaded from NHANES and will be analyzed to test the hypotheses that: 1) higher levels of cardiorespiratory fitness in adolescents are associated with lower ALT levels in adolescents;and 2) adolescents meeting the FITNESSGRAM(R) standards for acceptable cardiorespiratory fitness for adolescent boys and girls will have lower ALT levels than adolescents who do not meet the standards. The research proposed in this study carries high public health significance because it will begin to provide necessary information to understand the associations of physical activity, cardiorespiratory fitness and NAFLD in adolescents. This information may help promote policies and physical activity programs designed to reduce NAFLD in youth. PUBLIC HEALTH RELEVANCE: Non-alcoholic fatty liver disease (NAFLD) is a growing public health concern, particularly in sedentary, obese youth. Because NAFLD is associated with death from cardiovascular disease in adults, it is a public health priority to start prevention and treatment at an early age. This study will examine the associations of adolescent physical activity, physical fitness and NAFLD to help provide information for promoting policies to increase physical activity and reduce NAFLD in youth.